Create variable with string and rand

i want to create variable with string and random value
the result i want:

it is possible to concat rand in variable or

i have to define 5 variables as rand and concat
like this
http-request set-var(txn.vala) rand
http-request set-var(txn.valb) rand

http-request set-var(txn.myquery) str(),concat(a=,txn.vala),concat(&b=,txn.valb),concat(&c,txn.valc),concat…
of course without lua

thanks you for your help